Injection devices are known for the self administration of a medicament by patients. For example, those suffering from diabetes may require regular injections of insulin, others may require regular injections of a growth hormone. Injection devices allow the patient to select a dose and to administer that dose. It is known to automate this process so that a user need only press a button and the injection device will dispense a selected dose of medicament. This relieves the patient of the task of controlling the amount dispensed while manually expelling the medicament from the injection device. This is a particular problem for the elderly, the infirm, those suffering from vision difficulties and those suffering from diabetes related problems which impair their faculties.
The medicament is typically contained within a cartridge located within the injection device. The cartridge has a bung or piston at one end which is driven towards a second end of the cartridge to expel the medicament for the injection device. It is a problem that injection devices should be small enough to fit into a jacket pocket or a hand bag without difficulty. At the same time, the injection device must be of a size that enables a piston or the like used to drive the cartridge and to be fully withdrawn from the cartridge to allow for replacement of the cartridge.
The object of the invention is to provide a drive mechanism for an injection device and an injection device with such a drive mechanism by which these conflicting requirements are fulfilled.